Photodetectors are of interest in a wide variety of fields including optical communications, image sensing and instrumentation. Because of their low dark current and high responsivity, quantum well infrared photodetectors (QWIPs) offer considerable promise as high speed infrared photodetectors.
A particularly advantageous QWIP structure is disclosed in U.S. Pat. No. 4,894,526 to C. G. Bethea et al entitled Infrared Radiation Detector. In one embodiment the QWIP comprises a plurality of GaAs/AlGaAs quantum wells having a single bound state for electrons. Incident infrared radiation excites electrons from their bound state into the conduction band where they contribute to collected current.
A limitation of prior art QWIPs is that the quantum well regions are typically fabricated as mesa structures arising three or more micrometers from the underlying n.sup.+ semiconductor contact layer. The mesas present an extreme topography which is not readily compatible with integration of other electronic devices on the same substrate. Moreover the side walls expose the active area of the device to the degrading effects of contamination and radiation. Because the mesa topography is not suitable for integration, the conventional QWIPs must be fabricated in dimensions sufficiently large to be aligned and bonded to other substrates containing integrated circuits. Although smaller mesas would be desirable, this bonding requirement limits the minimum mesa diameter to about 30 micrometers. Accordingly, there is a need for a QWIP structure which eliminates mesa topography.